Aluminum base alloy



United States Patent ABSTRACT OF THE DISCLOSURE The present inventionrelates to new and improved aluminum base alloys containing magnesium.More particularly, the present invention resides in aluminum base alloyscontaining from 5.5 to 10% magnesium and characterized by improvedphysical properties such as high strength and stress corrosionresistance.

The advantages to be derived from alloying magnesium with aluminum basealloys were recognized very early in the development of aluminumtechnology. Consequently, the aluminum-magnesium series of alloys is oneof the oldest used commercially.

The development of inert gas shield are methods in welding in recentyears has stimulated additional interest in sheet and plate of thestronger alloys in this series. In addition, the excellent physicalproperties of these alloys in welded structures is well recognized, suchas the high yield strength obtainable Without heat treatment, goodweldability and good ductility.

Attempts have frequently been made to increase the magnesium content ofthe aluminum base alloys in wrought form up to 10%. These attempts,however, have not resulted in commercialization of aluminum base alloyscontaining more than 5.5% magnesium because of inherent problems ofstress corrosion susceptibility of these alloys in the cold workedcondition. Therefore, at the present time there are no satisfactorycommercially available aluminum base alloys containing more than 5.5%magnesium in cold worked tempers.

It is, therefore, highly desirable to develop such alloys due to theexcellent physical properties which they prom ise, such as light weight,high strength levels equivalent to those of mild steel, excellentductility and weldability. However, the inherent problems of stresscorrosion susceptibility of these alloys in the cold worked tempers mustbe overcome. In other words, aluminum base alloys containing greaterthan 5.5% magnesium are generally not used at present commercially instrain hardened tempers because of their great susceptibility to stresscorrosion cracking.

Accordingly, it is a principal object of the present invention toprovide new and improved aluminum base alloys containing greater than5.5% magnesium.

It is a further object of the present invention to provide alloys asaforesaid which are characterized by excellent physical characteristics,such as high yield strength, good weldability and good ductility.

It is a still further and particular object of the present invention toprovide alloys as aforesaid which overcome the great susceptibility ofthis type of alloy to stress corrosion cracking.

It is a further object of the present invention to provide a convenientand expeditious process for obtaining the aforesaid alloys.

Further objects and advantages of the present invention will appearhereinafter.

It has been found in accordance with the present invention that theforegoing objects and advantages may be readily attained by providing analuminum base alloy consisting essentially of the following composition:from 5.5 to 10% magnesium; from 0.05 to 0.3% chromium; from 0.10 to0.80% cobalt; from 0.10 to 0.60% copper; and the balance essentiallyaluminum. In the preferred embodiment, the present invention employsfrom -6 to 8% magnesium; from 0.1 to 0.2% chromium; from 0.15 to 0.60%cobalt; from 0.15 to 0.40% copper; and the balance essentially aluminum.

It has been found surprisingly that the foregoing alloy in the foregoingcritical compositional ranges overcomes the heretofore noteddisadvantages of the art. Particularly surprising is the unusual stresscorrosion resistance of the alloy of the present invention. For example,environmental stress corrosion tests were run in a rigorous atmospherewith the following results: an alloy of the present invention containing7.2% magnesium, 0.15% chromium, 0.56% cobalt, and 0.26% copper, balanceessentially aluminum, was subjected for a period in excess of one yearwith no stress corrosion failures, with the test still proceeding;whereas substantially the same alloy without the cobalt and copperexhibited stress corrosion failure in 300 days; and substantially thesame alloy without the cobalt, copper and chromium exhibited stresscorrosion failure after days of exposure.

Furthermore, the present invention provides a process for obtaining theforegoing improved alloys which comprises: (A) providing an aluminumbase alloy consisting essentially of the foregoing materials in theforegoing critical compositional ranges; (B) hot rolling said alloy at atemperature of from 450 to 950 F. to :a gage of less than 2"; and (C)cold rolling said alloy. The alloy is preferably cold rolled tointermediate gage, although it may be cold rolled to final gagedirectly, if desired. The amount of cold rolling reduction is limited bymill capability.

Prior to the hot rolling step, it is preferred to provide a heattreatment or homogenization step at from 850 to 975 F. for from 5 to 30hours and preferably 10 to 16 hours.

Preferably the alloy is stabilized after cold rolling by holding at atemperature of from 200 to 450 F. for at least 15 minutes and preferably1 to 4 hours; however, the alloy may, if desired, be utilized in thecold rolled condition.

In the preferred embodiment the following additional process steps areperformed after cold rolling but before stabilizing in the event thatmore cold rolling reduction is necessary or desired or if the materialis required in the annealed temper: (D) annealing at :a temperature of500 to 1000 F., and preferably 650 to 950 F. for at least 5 minutes andpreferably at least 60 minutes; and (E) cooling said alloy, preferablyat a rate of 50 F. per hour or less, to room temperature. After theintermediate anneal, the alloy may again be cold rolled to the desiredtemper. This sequence of annealing, cooling, and cold rolling may berepeated as often as necessary. In addition, as

3 indicated above, the alloy may be stabilized after the final cold rollby holding said alloy at a temperature of 200 to 450 F. for at least 15minutes and preferably 1 to 4 hours.

It is further preferred that all thermal treatments, including thepreliminary heat treatment or homogenization treatment, the hot rollingstep, and subsequent interannealing of the hot rolled material, befollowed by a slow, controlled cool down rate of 500 F. per hour or lessto room temperature and preferably 50 F. per hour or less.

The process of the present invention provides an improvide alloy in thecold rolled tempers. The greatest improvements are provided when thealloy is subjected to two (2) or more cold rolls with intermediateanneals and in particular when the alloy is in the cold rolled plusstabilized condition. When the alloy is in the cold worked temper it ischaracterized by a minimum yield strength of 45,000 p.s.i., with yieldstrengths generally on the order of 48,000 to 60,000 p.s.i., a minimumtensile strength of 55,000 p.s.i. and generally from 60,000 to 75,000p.s.i. and a minimum elongation of 6% with elongations generally on theorder of 8 to 10%. After recovery, i.e., after the holding orstabilizing step, the alloy is characterized by a minimum yield strengthof 35,000 p.s.i. and generally from 37,000 to 55,000 p.s.i., a minimumtensile strength of 50,000 p.s.i. and tensile strength generally from56,000 to 70,000 p.s.i. and a minimum elongation of 12% with elongationsgenerally from 15 to percent.

It is also quite surprising that the fully annealed properties of thealloys of the present invention are quite high as compared toconventional aluminum-magnesium alloys, for example, the fully annealedproperties of the alloys of the present invention are: yield strength,from 20,000 to 30,000 p.s.i., tensile strength, from 45,- 000 to 55,000p.s.i., and elongation, from 20 to 30%.

The foregoing characteristics of the alloys of the present invention areparticularly surprising and represent a considerable improvement overconventional alloys of this type.

In addition, the cold rolled properties, both before and after recovery,are characterized by good corrosion resistance and excellent stresscorrosion resistance. These alloys, surprisingly, will not fail both incold worked and stabilized tempers under prolonged exposure in theambient temperature range, i.e., up to 180 F.; whereas, all other alloysof this type will catastrophically fail under these conditions. Thealloys of the present invention in the cold worked and stabilizedtempers have been shown to hold up for one year and longer in rigorous,natural environmental testing, with the test still proceeding withoutfailure.

The melting and casting of the alloys is not particularly critical. Thealloys may be melt and east by any conventional method, such as, forexample, the direct chill or tilt mold method.

The alloy of the present invention also exhibits good physicalproperties as a cast product and will show a significant strengthadvantage over conventional aluminum-magnesium alloys. For this use, thealloy may be cast into final shape using conventional sand and permanentmolding techniques.

In addition to the foregoing critical alloying additions, the presentinvention contemplates small amounts of additional alloying ingredientswhich will not deleteriously affect the properties of the alloy and may,in fact, enchance a given physical property. For example, indium;gallium; cadmium; lithium; manganese; zinc; thorium; boron; tellurium;misch metal; germanium; and hafnium.

In addition to the foregoing alloying additions, naturally the presentinvention contemplates the use of the normal impurity levels common tocommercial grade aluminum. However, impurity ranges should be maintainedwithin the following limits: iron, up to 0.50%; silicon, up to 0.50%;manganese, up to 0.35%; zinc, up to 0.2%; titanium, up to 0.15%;beryllium, up to 0.02%; and others in total up to 0.2%. In fact, it maybe desirable to add one or more of the foregoing materials in order toenhance a given property, for example, castability or to minimizestaining during annealing. Beryllium is a preferred alloying addition inamounts from 0.0005 to 0.02%, and optimally from 0.001 to 0.005%. V

The present invention will be more readily understandable from aconsideration of the following illustrative examples.

EXAMPLE I Ingots, designated alloy A, were prepared of the alloy of thepresent invention in a conventional manner summarized as follows:melting and alloying were carried out in an induction heating furnace.The melt was stirred after each alloying addition and just beforeflexing, with the melt being degassed by gaseous chlorine fluxing at arate of 3000 cc. per minute for 15 minutes. The melt temperature wasmaintained at 1350 to 1360 F. The charge was then bottom poured usingstandard, direct chill casting techniques at an average casting speed of3.5 to 4.0" per minute on a 3" x 6" mold section.

The alloys of the present invention were prepared in this manner and hadthe following composition:

Percent Magnesium 7.2 Iron 0.29 Silicon 0.12 Copper 0.26 Titanium 0.13Beryllium 0.002 Chromium 0.15 Cobalt 0.56

EXAMPLE II For comparative purposes, two alloys were prepared in thesame manner as in Example I to have the following composition:

Comparative alloy B EXAMPLE III The alloys prepared in Examples I and IIwere homogenized at 950 to 975 F. for 16 hours at temperature followedby slow cooling at a rate slower than 50 F. per hour to roomtemperature. The ingots were then hot rolled at 675 F. to 0.172 gage,followed by slow cooling at the above rate to room temperature, followedby cold rolling to 0.086" gage. The alloys were then interannealed at800 F. for 4 hours followed by slow cooling to room temperature at theabove rate followed by cold rolling to 0.060" gage. The alloys were thenCut up for testing with the following results:

The alloys treated in accordance with Example 111 in 0.060" gage werestabilized by heating to 300 F. and holding at that temperature for fourhours. The alloys were then cut up for testing with the followingresults:

This example shows the surprising stress corrosion resistance of thealloys of the present invention. In this example various samples Weresubjected to environmental stress corrosion tests run in a rigorousatmosphere. The test consisted of exposing a pro-stressed sample to theelements on the beach at Daytona Beach, Fla, for a period of time untilthe sample showed failure by stress corrosion cracking. The sample Waspro-stressed 'by bending in the shape of a letter U. Normally thefailure by stress corrosion cracking was first exhibited at the apex ofthe sample.

Alloys A, B and C were tested, with each sample being tested in thefollowing conditions: (1) five samples in the cold worked conditionafter the treatments of Example III; (2) five samples in the stabilizedcondition after the treatments of Example IV; and (3) five samples inthe sensitized condition, a condition designed to exaggerate stresscorrosion susceptibility. The sensitization treatment consisted ofheating to 300 F., holding for 24 hours and cooling to room temperature.The results are shown in the following table:

TABLE III Alloy Condition Time to Failure by Stress Corrosion CrackingCold Worked... No failure after 15 months and still testing. StabilizedDo. Sensitized N :2 failure after 10 months and still esting. ColdWorked All samples failed from 111 to 185 days. Stabilizei- All samplesfailed 1mm 27 to 55 days.

Seusitizei. -1 All samples failed from 24 to 35 days.

Co1dWorked No failure after 12 months and still testing. 0 Stabilized Nofailure after 14 months and still testing. C sensitized All samplesfailed from 100 to 300 days.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the cope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

1. An aluminum base alloy having good stress corrosion resistanceconsisting essentially of: from 5.5 to 10% mag nesium; from 0.05 to 0.3%chromium; from 0.10 to 0.80% cobalt; from 0.10 to 0.60% copper; and thebalance essentially aluminum.

2. An aluminum base alloy having good stress corrosion resistanceconsisting essentially of: from 6 to 8% magnesium; from 0.1 to 0.2%chromium; from 0.15 to 0.60% cobalt; from 0.15 to 0.40% copper; and thebalance essentially aluminum.

3. An alloy according to claim 1 containing beryllium in an amount from0.0005 to 0.02%.

4. An al-loy according to claim 2 containing beryllium in an amount from0.001 to 0.005%.

5. An alloy according to claim 1 containing iron in an amount up to0.50%, silicon in an amount up to 0.50%, manganese in an amount up to0.35%, zinc in an amount up to 0.2%, titanium in an amount up to 0.15%,beryllium in an amount up to 0.02%, and all others in total up to 0.2%.

References Cited UNITED STATES PATENTS 2,240,940 5/1941 Nock 148-12.7 X2,336,512 12/ 1943 Stroup 147 2,628,899 2/ 1953 Willrnore 75-1472,841,512 10/1956 Cooper 148ll.5 3,232,796 2/ 1966 Anderson l48--12.7

DAVID L. RECK, Primary Examiner. H. F. SAITO, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,546,375 October 10, 1967 George J. Jagaciak It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column 3, lines 12 and 13, "improvide" should read mproved line 68,"enchance" should read enhance olumn 4, line 52, before "0.004" insertless than line 53 efore "0.001" insert less than line 54, before "0.004"nsert less than Signed and sealed this 9th day of December 1969.

est:

'ard M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

:sting Officer Commissioner of Patents

1. AN ALUMINUM BASE ALLOY HAVING GOOD STRESS CORROSION RESISTANCECONSISTING ESSENTIALLY OF: FROM 5.5 TO 10% MAGNESIUM; FROM 0.05 TO 0.3%CHROMIUM; FROM 0.10 TO 0.80% COBALT; FROM 0.10 TO 0.60% COPPER; AND THEBALANCE ESSENTIALLY ALUMINUM.